In modern communication systems, it is important to be spectrally efficient so that different channels can operate at the same time on closely spaced frequency bands without interfering with each other. To keep interference to the minimum, most communication standards require the transmitted signal to meet a tight spectral mask. Typically, the signal needs to be filtered to meet the spectral requirements. Filtering can be achieved by using analog filter, digital filter, or a combination of both. Since it is harder and more expensive to implement sharp filters in analog circuits, most modern communication systems use digital filters to do most of the filtering work.
To achieve output that meets the required spectral mask, the input signal often needs to be interpolated to generate an output signal that is band limited in the frequency domain and smoothly shaped in the time domain. Interpolation is generally done by upsampling the signal by inserting points and then filtering it with a lowpass filter. Typically, 0's are inserted into the signal and then the upsampled, 0-inserted signal is convolved with the impulse response of a lowpass filter. The resulting output is band limited and smoothed.
FIG. 1 is a block diagram of a transmitter that includes upsampling and filtering. Input data 101 is encoded by encoder 102. The encoded signal is then upsampled by upsampler 103. The upsampled signal is then filtered by a finite impulse filter (FIR) 104 to produce a better shaped waveform at a higher frequency. The signal is then converted to an analog signal by a digital to analog converter (DAC) 105. Interpolator 106 includes the upsampler 103 and FIR 104. The output of DAC 105 is generally sent to a mixer and then transmitted.
FIG. 2A is a block diagram of one implementation of interpolator 106 in FIG. 1. The sampling rate of the input signal is increased to a higher frequency by upsampler 201. 0's are inserted in the upsampled points where original input values are not available. FIR filter 202 then filters the signal to make it band limited and smooth. In practice, when the filter is complex, its circuitry becomes large in area and therefore expensive. Also, because the large filter is running at the upsampled frequency, it consumes more power. It would desirable to have filter designs that would reduce area and power consumption.
FIG. 2B is a block diagram of a filter design that uses cascading upsamplers and filters to reduce circuitry area and power consumption. The signal is first upsampled to an intermediate frequency by first upsampler 211. The signal is then filtered by an FIR filter 212. Another upsampler 213 upsamples the signal and filter 215 again filters the signal until the desired output is achieved. Additional upsampling and filtering stages may be included. Such a cascading filter can achieve the same results as the one in FIG. 2A, but with smaller total area and power consumption. Although this design is an improvement, there is still a need for a system that provides a further reduction in area, complexity and power.